Relative electrode potentials

Knowledge of the Volta potential of a metal/solution interface is relevant to the interpretation of the absolute electrode potential. According to the modem view, the relative electrode potential (i.e., the emf of a galvanic cell) measures the value of the energy of the electrons at the Fermi level of the given metal electrode relative to the metal of the reference electrode. On the other hand, considered separately, the absolute value of the electrode potential measures the work done in transferring an electron from a metal surrounded by a macroscopic layer of solution to a point in a vacuum outside the solotion. ... 

In summary, sodium reacts with water and copper does not in consequence of their relative electrode potentials. 

Pig. 4-23. Measurement of relative electrode potential by a potentiometer M = test electrode M° = reference electrode F = potentiometer = Fermi level of electron in electrodeM = Fermi level of electrons in terminal A E = relative electrode potential. 

Fig. 4-24. Electron energy levels for electrode potential relative to a reference electrode E = electrode potential (absolute) E = relative electrode potential Ps = outer potential of electrolyte solution of test electrode = outer potential of electrolyte solution of reference...

The relative electrode potential nhe referred to the normal (or standard) hydrogen electrode (NHE) is used in general as a conventional scale of the electrode potential in electrochemistry. Since the electrode potential of the normal hydrogen electrode is 4.5 or 4.44 V, we obtain the relationship between the relative electrode potentiEd, Ema, and the absolute electrode potential, E, as shown in Eqn. 4-36 ... 

Figure 4-25 compares the relative electrode potential, Etna, both with the (absolute) electrode potential, E, and with the real potential, of electrons in the... 

Fig. 4-25. Comparison between the real potential a.M

The relative work function and the relative electrode potential of electrodes in aqueous solutions and in inactive gases can be measured by a vibrating capacitor technique called Kelvin s method [Samec-Johnson-Doblhofer, 1992]. The Kelvin method estimates the difference in the work function between a test electrode and a Kelvin probe (KF) by measuring the applied voltage V at which the difference in the outer potential ij s- l KP between the test electrode and the Kelvin probe becomes zero (V = liJs - i Kp) as shown in Pig. 4—28. 

Fig. 4-28. Schematic layout of Kelvin s vibrating capacitor method to measure relative electrode potential of (a) electrode immersed in aqueous solution and of (b) electrode emersed from solution KP = Kelvin s probe 4 s = outer potential of aqueous solution 4>kp = outer potential of Kelvin s probe V and = applied voltages to cancel out a difference in the outer potential.

TABLE 6-L Die standard equilibrium potentials for redox electrode reactions of h rdrated redox particles at 25 C nhe = relative electrode potential referred to the normal hydrogen electrode. [Handbooks of electrochemistry.]... 

We have seen in Section 5.2 that one can determine the relative electrode potential by measuring cell voltage. To form a series of relative electrode potentials, one has to select a reference electrode and standard conditions of components of an electrode/ electrolyte interphase. 

The standard hydrogen electrode (Fig. 5.5) is chosen as the reference electrode when a series of relative electrode potentials is presented. By convention, the standard potential of this electrode is set to zero. Connecting this reference electrode with other electrodes into a cell, one can determine a series of relative values of electrode potentials (potential differences across interphases). For example, consider the cell shown in Figure 5.9. This cell can be represented schematically in the following way ... 

The reducing powers of metals parallel their relative electrode potentials, i.e. potentials developed when the metal is in contact with a normal solution of its salts. The potential of hydrogen being equal to 0, the potentials or electrochemical series of some elements are as shown in Table 4. 

Table 4 Physical constants and relative electrode potentials of some metals...

Electrode potentials are measured by coupling electrodes in a cell, the second electrode of which is a certain constant one. The one that is chosen is usually a hydrogen electrode, as described previously.21 It is then the potentials of such cells that are called relative electrode potentials or potentials of electrodes on the standard hydrogen scale. What is meant is that by an arbitrary convention, these particular cell potentials are no longer called cell potentials but relative electrode potentials, and, indeed, the word relative is often dropped because those in the know realize what is meant... 

When one refers to the relative electrode potentials of a number of systems on the hydrogen scale and then equates these with the potentials of the cells made up of the systems (the electrode-solution systems concerned) in combination with a hydrogen electrode, the situation is that one is arbitrarily taking the constant... 

Of course, if this constant is taken as zero for all the metals with which the potentials of the other systems are compared, there will be no effect of this constant, but one may never forget that the relative electrode potential, to which reference is so often made, is in fact not a metal-solution potential difference. 

Fig. 6.35. The meaning of the relative potential difference across the Cu/CuS04 interface, i.e.. the relative electrode potential, (a) The electrochemical cell corresponding to the relative potential difference, (b) The relative potential difference includes a platinum-copper contact potential and the unknown potential difference across the SHE, apart from the absolute potential difference across the Cu/CuS04 interface.

Define the following terms used in Section 6.3 (a) electrochemical cell, (b) ideally nonpolarizable and polarizable interfaces, (c) relative electrode potential, (d) outer potential, (e) inner potential, (1) surface potential, (g) image forces, (h) Coulombic forces, (i) electrochemical potential, (j) chemical potential, (k) electron work function, (1) just outside the metal, and (m) absolute potential. (Gamboa-Aldeco)... 

Table E.l is a list of reduction potentials of several reactions given in the relative-electrode potential scale. Write the potentials of the same reactions in... 

This equation is virtually identical to the Jdnetically deduced version of Eq. (7.40). However, it is not yet formally identical with that of Nernst, which was deduced long before the concept of a Galvani potential difference (MdS< >) across the metal/solution interface was introduced (Lange and Misenko, 1930). Nernst s original treatment was in terms of the electrode potential and symbolized by V. It is possible to show (see Section 3.5.15) that for a given electrode, M S< > - V + const. (i.e., the factors that connect the measured electrode potential to the potential across the actual interface) do not depend on the activity of ions in the solution. Hence, using now the relative electrode potentials, Vt in place of the absolute potentials ,... 

Sacrificial anode — is a piece of metal used as an anode in electrochemical processes where it is intended to be dissolved during the process. In -+ corrosion protection it is a piece of a non-noble metal or metal alloy (e.g., magnesium, aluminum, zinc) attached to the metal to be protected. Because of their relative -+ electrode potentials the latter is established as the -+ cathode und thus immune to corrosion. In -+ electroplating the metal used as anode may serve as a source for replenishing the electrolyte which is consumed by cathodic deposition. The sodium-lead alloy anode used in the electrochemical production of tetraethyl lead may also be considered as a sacrificial anode. 

In electrochemistry, it is usual to measure potentials with respect to a stable and reproducible system, known as - reference electrode. For the vast majority of practical electrochemical problems there is no need to determine - absolute potentials. However, this is necessary in cases where one wants to connect the relative electrode potential with the absolute physical quantities of the system, like electronic energies, as is the case of the work function. It is possible to convert all relative values of electrode potential to absolute-scale values and to electronic energies. For aqueous systems the - standard hydrogen electrode potential corresponds to -4.44 V in the physical scale taking electrons at rest in vacuum as reference and the absolute potential is given by the relation T(abs) = T(SHE) + 4.44 [vii]. 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

For most purposes, it suffices to have relative electrode potentials in order to gain the thermodynamic information being sought. This is a fortunate situation since the electrode potential for a half reaction cannot be measured. There are instances where it would be very desirable to be able to estimate the energy of such a reaction. Since the gas phase ionization reactions (39) and (40) can be studied, it is possible to relate the gas phase energetics, the ioniza-... 

For relative electrode potential data to be widely applicable and useful, we must have a generally agreed-upon reference half-cell against which all others are compared. Such an electrode must be easy to construct, reversible, and highly reproducible in its behavior. The standard hydrogen electrode (SHE) meets these specifications and has been used throughout the world for many years as a universal reference electrode. It is a typical gas electrode. 

Any sign convention must be based on expressing half-cell processes in a single way—that is, either as oxidations or as reductions. According to the lUPAC convention, the term electrode potential (or, more exactly, relative electrode potential ) is reserved exclusively to describe half-reactions written as reductions. There is no objection to the use of the term oxidation potential to indicate a process written in the opposite sense, but it is not proper to refer to such a potential as an electrode potential. 

Relative electrode potential The potential of an electrode with respect to another (ordinarily the standard hydrogen electrode or saturated calomel electrode). 

Single-electrode potential Synonymous with relative electrode potential. 

Half-cell reactions cannot be stndied in isolation all that can be measnred is the difference in potential (A ) when two half-cells are linked to form an electrochemical cell. Relative electrode potentials for half-cells are obtained by reference to a standard half-cell, the hydrogen electrode that is assigned an Eq of 0.0 V. 

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